<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

    <script>
        var str = '';

        for(var b = 1;b < 11;b ++){

            str += 'helloworld<br>';

        }

        document.write(str);

        var i = 12;

        var sum =i++ + ++i + ++i*2 + i-- + i--;

        document.write(sum + " " + i);

    </script>

</body>
</html>